A method for simultaneously removing sulfur dioxide (SO2) and nitrogen oxides (NOx) from coke oven flue gas using steel slag slurry was proposed. Due to the high removal efficiency of SO2, the effects of operation conditions on the removal of NOx were investigated emphatically. The results showed that the removal efficiency of NOx increased with an increase in steel slag slurry concentration, NOx oxidation ratio, steel slag slurry pH, and NOx and SO2 inlet concentrations and a decrease in reaction temperature. The removal efficiencies of SO2 and NOx could reach 100 and 83.4%, respectively, by optimizing the operation conditions. The mechanism of NOx removal using steel slag slurry was investigated. The results showed that (MgO)(0.841).(MnO)(0.159) (RO phase, a CaO-FeO-MnO-MgO solid solution) in steel slag was decomposed by H+ and produced Mg2+ and Mn2+. Furthermore, Mn2+ in steel slag slurry could promote the absorption of NO2 via redox reactions with the generation of solid Mn3O4 and MnO(OH).